GENERAL

This test method describes how to determine the dew point
using a sling psychrometer. A psychrometer measures the ambient air temperature
(dry bulb temperature) and the wet bulb temperature. From the dry bulb and wet
bulb temperature the dew point is calculated. The dew point is the temperature
at which moisture in the air will start condensing.

When painting
structural steel, specifications may require the dew point to be measured. This
is to ensure that no moisture is condensed on the surface to be painted.

APPARATUS

1. Sling psychrometer

2. Replacement
thermometer

3. Replacement wicks

INSTRUMENT OPERATION

1. Before
using, take the end cap off the psychrometer and fill the reservoir with water.
Be sure the wick is saturated with water and covers the mercury reservoir on
the wet bulb thermometer. Be sure the mercury reservoir on the dry bulb
thermometer is dry.

2. Pull the tube clear of the body so the body can
swivel.

3. Holding the tube, whirl the body two to three
revolutions per second.

5. Immediately read the wet bulb thermometer and then
the dry bulb thermometer.

6. Figure
the difference between the dry bulb and wet bulb temperature. On Table 1,
locate the column, which is headed with this temperature difference. Then
locate the row labeled with the appropriate dry bulb temperature. The value at
the intersection of the column and row gives the dew point.

7. The
dew point should be at least 5°F less than the surface temperature of the steel
being painted.

EXAMPLE

If the dry bulb
temperature is 71°F and the wet bulb temperature is 64°F, the difference
between the dry bulb and wet bulb is 7°F. At the top of Table 1, locate the
column headed with 7. Follow this column down to the row headed with the dry
bulb temperature of 71°F. The intersection of the column and row gives the
number 60, which is the dew point in degrees Fahrenheit. Add 5 degrees to the dew
point to get 65°F. Sixty-five is the minimum surface temperature of the steel
before painting should begin to ensure that there is no condensation on the
surface.

NOTES

1. The surface and air temperature will vary depending
on the location of the temperature measurements. For instance, a beam exposed
to direct sunlight will be much warmer than a beam in the shade. Therefore if
painting work is done in the shade, temperature measurements should also be
done in the shade.

2. Moisture problems are most likely to be encountered
in the morning, so temperature measurements should be made in the morning
before painting begins.

REPORTING RESULTS

The following
information should be reported: Time, dry bulb temperature, wet bulb
temperature, dew point, and surface temperature.